The main rotating assemblies of gas turbine engines comprise compressors and turbines, rotating on shafts. As with any rotating assembly, any imbalance in these assemblies will give rise to vibration and stresses. The magnitude of these stresses increases as the square of the rotational speed.
Rotational speeds in gas turbine engines vary with the type of engine and the conditions of operation, but speeds of several thousand revolutions per minute are common. Because of these high rotational speeds, even slight unbalance in the main rotating assemblies can quickly give rise to damaging stresses, and so very accurate balancing of the assemblies is essential.
Two common ways of balancing a rotor disc or drum of a gas turbine engine are
i) by providing additional material (a “balance land”) on the rotor drum. This is then selectively machined away during manufacture to achieve the desired balance adjustment. The balance land is essentially parasitic mass. It cannot be assumed to carry any loads, as there is no guarantee it will be there in the final part. Conversely, for stress purposes it must be assumed that all the mass is present, in case none of it has to be machined away. The balance land is therefore an undesirable feature;
ii) by adding additional mass selectively to a component or assembly, in the form of, for example, screwed or riveted plugs, heavy wire, balancing plates or nuts. This method also adds parasitic mass to the component, and has the further disadvantage that the balancing masses may become detached from the component and cause damage elsewhere in the engine.
Conventionally, rotor discs and drums are connected to neighbouring components by means of bolted flanges. As well as the bolt holes, such rotors typically have stress defender holes, which help to reduce the increased stresses found at stress concentration features around the bolt holes.